A conventional wrench having a box end is used for a long time and the box end has a polygonal inner periphery so that an object such as a nut or a screw head can be engaged with the polygonal inner periphery and rotated with the wrench. An inherent shortcoming of the conventional box end wrench is that the box end has to be removed from the nut after being rotated an angle and re-cap to the nut at desired position. This shortcoming is more obvious when other objects are located close to the nut and the user is suffered by the repeated actions of removing the box end from the nut and re-mounting the box end to the nut.
The present invention intends to provide a wrench, especially to a box end of a wrench wherein a plain surface is defined in an end of the inner periphery of the box end and a spring member is engaged with the box end so that the wrench can be freely rotated regardless of the object when the object is located in the position enclosed by the plain surface.